Explosions and Blast Injuries

Explosions and blast injuries are fortunately very uncommon in most countries. With the exception of working as a paramedic in the defence forces during active service, it is unlikely that you will ever attend a patient suffering with severe blast injuries after an explosion. Unfortunately, although very infrequent, terrorists acts do occur as well as accidents involving explosions, so it is important to have a basic understanding of explosions and blast injuries. 

It is important to understand that explosions cause significant and complex injuries to people and even a minor explosion, such as one caused by the reaction of a heated aerosol can has the potential to cause significant injuries often hidden internally.

What is an Explosion?

An explosive material is a reactive substance that contains a great amount of potential energy that can produce an explosion if released suddenly, usually accompanied by the production of light, heat, sound, and pressure. An explosive charge is a measured quantity of explosive material.

This potential energy stored in an explosive material may be one of the following 3 types:  chemical energy, such as nitroglycerin; pressurized gas, such as a gas cylinder or aerosol can, and nuclear energy, such as in the fissile isotopes uranium-235 and plutonium-239. It is also important to understand that uranium may be used in conjunction with traditional chemical explosions to create a dirty bomb.

The most common of these three types of explosions seen by paramedics today is pressurized gas explosion often as the result of burning of aerosol cans or gas cylinders for barbeques. It is important to remember that not all explosions are intentional terrorist acts.

Explosive materials may be categorized by the speed at which they expand. Materials that detonate (explode faster than the speed of sound) are identified as “high explosives” and materials that deflagrate are identified as “low explosives.” Explosives may also be categorized by their sensitivity. Sensitive materials that can be initiated by a relatively small amount of heat or pressure are primary explosives and materials that are relatively insensitive are secondary or tertiary explosives.

Mechanism of Injury of Explosions

Primary injuries are caused by blast overpressure waves, or shock waves. These are especially likely when a person is close to an explosion. The ears are most often affected by the overpressure, followed by the lungs and the hollow organs of the gastrointestinal tract. Gastrointestinal injuries may present after a delay of hours or even days.  Injury from blast overpressure is a pressure and time dependent function. By increasing the pressure or its duration, the severity of injury will also increase.

In general, primary blast injuries are characterized by the absence of external injuries; thus internal injuries are frequently unrecognized and their severity underestimated. Although poorly understood, there is a general agreement that spalling, implosion, inertia, and pressure differentials are the main mechanisms involved in the pathogenesis of primary blast injuries.

Secondary injuries are caused by fragmentation and other objects propelled by the explosion. These injuries may affect any part of the body and sometimes result in penetrating trauma with visible bleeding. At times the propelled object may become embedded in the body, obstructing the loss of blood to the outside. However, there may be extensive blood loss within the body cavities. Fragmentation wounds may be lethal and therefore many anti-personnel bombs are designed to generate fragments.

Most casualties are caused by secondary injuries. Some explosives, such as nail bombs, are deliberately designed to increase the likelihood of secondary injuries. In other instances, the target provides the raw material for the objects thrown into people, e.g., shattered glass from a blasted-out window or the glass facade of a building.

Tertiary injuries are caused by the displacement of air by the explosion which creates a blast wind that can throw victims against solid objects. Tertiary injuries may present as some combination of blunt and penetrating trauma, including bone fractures and coup/ contre-coup injuries to the brain.

Quaternary injuries are all other injuries not included in the first three classes. These include flash burns, crush injuries and respiratory injuries.

High-order explosives produce a supersonic overpressure shock wave, while low order explosives deflagrate (subsonic combustion) and do not produce an overpressure wave. A blast wave generated by an explosion starts with a single pulse of increased air pressure, lasting a few milliseconds. The negative pressure (suction) of the blast wave follows immediately after the positive wave. The duration of the blast wave, i.e., the time an object in the path of the shock wave is subjected to the pressure effects, depends on the type of explosive material and the distance from the point of detonation. The blast wave progresses from the source of explosion as a sphere of compressed and rapidly expanding gases, which displaces an equal volume of air at a very high velocity.

The severity of the blast injury is subject to the following criteria: the peak of the initial positive barometric pressure, the duration of the overpressure, the degree of focussing due to a confined space, the medium in which it explodes.

The most important things to remember when you attend an explosion or treat a patient with blast injuries are: don’t become a victim yourself, look for dangers, and get the patient and yourself out of there; transport urgently to definitive surgical care, these patients are going to have major injuries, whether they’re visible injuries or internal. Reassurance, oxygen therapy, burns dressings, IV access and analgesia should all be provided en route to hospital. Traumatic amputations quickly result in death, and are thus rare in survivors, and are often accompanied by significant other injuries – if you are on scene early enough, arterial tournequets may just save a life.